Some heat exchangers are operating with a vertical two-phase flow of at least one of the fluids, which comprises a gas phase and a liquid phase typically distributed as droplets in the gas phase. For example in catalytic reforming a preheating heat exchanger is used in which an upward flow of liquid naphtha and recycled gaseous hydrogen is heated by the effluent from the reforming reactor. The effluent comprises e.g. reformates and hydrogen.
The liquid phase of the fluid must be driven upwards by the gas phase of the fluid. Thus, in order to move the liquid phase upwards, on each droplet of the liquid phase the aerodynamic lifting force must be greater than the gravitational force or if the droplet is in contact with the wall greater than the sum of the gravitational force and the frictional force of the wall. The lifting force of the gas phase acting on the liquid phase must be larger than the gravitational force and possibly the frictional force acting on the liquid phase.
The lifting force is dependent on for example the fluid and its characteristics, the geometry of the flow path of the fluid through the heat exchanger and the operating conditions, such as temperature, pressure, flow rates, etc.
Sufficient lifting force can be obtained by e.g. having a high flow rate of the gas phase and a high temperature of the fluid. Also a heavy gas having a high molar mass in the gas phase increases the lifting force.
However, to obtain a high efficiency and a low pressure drop the temperature and molar mass should preferably be kept down. There are also constraints, such as pressure drop and compressor limitations, that incite operators to lower the gas flow rate. Thus, it is optimal to regulate the parameters such that lifting force is sufficient but not unnecessarily excessive. Further, it is important to obtain a good distribution of fluids over the heat exchanging area of a heat exchanger to improve the efficiency and avoid mechanical damages. Consequently, there is a need for optimizing the operating parameters such that a sufficient lifting force of the gas phase is obtained.